This application is submitted in response to RFA-AI-07-034. We have constructed a model of systemic infection of immunocompetent mice by chimeric HIV-1, EcoHIV. Our previous studies indicate that EcoHIV replicates in lymphocytes and macrophages in infected mice, infection in mice is sensitive to antiretroviral drugs, productive infection persists for months inducing immune responses, and HIV-1 DNA vaccination can block infection in mice. Preliminary results reported here show that sexual transmission of EcoHIV in mice is rapid and efficient. Our overall goal in this application is to develop the mouse infection system to investigate the mechanisms of sexual transmission of HIV-1 as a platform to test efficacy of candidate microbicides. The Specific Aims of are: 1) to optimize conditions for sexual transmission of EcoHIV in mice and evaluation of interventions. 2) to identify the cell types involved in sexual transmission of EcoHIV. 3) to test the inhibition of sexual transmission of EcoHIV by antiretroviral-based microbicides. 4) to determine the HIV-1 subtype dependence of sexual transmission and efficacy of antiretroviral based microbicides against different HIV-1 subtypes. 5) to determine whether combination administration of an HIV-1 DNA vaccine followed by a microbicide can prevent sexual transmission of subtype B EcoHIV. Chimeric HIV-1 will be transmitted to conventional, immunocompetent female mice by mating with males infected through inoculation. Virus burden in multiple organs will be measured by real- time PCR and productively infected cells will be identified by flow cytometry and confocal microscopy. Accomplishment of Aims 1-3 will provide a firm foundation for and justification to extend the model to Aims 4-5 in studies directly relevant to the current HIV-1 epidemic and realistic means to control it. HIV-1 infection continues to spread worldwide, primarily by sexual transmission. The public health community responded to this pandemic by research into microbicides, compounds that women can apply to prevent transmission of HIV-1 during intercourse. Unfortunately, there is no simple way to determine which of many microbicides being developed actually blocks HIV-1 transmission before women begin their use. Some of the first to be tested by women in clinical trial actually increased HIV-1 transmission. This application is designed to develop a system for preclinical testing of microbicides in mice to determine their ability to reduce or prevent sexual transmission of HIV-1. We have shown that a form of HIV-1 that we genetically engineered to infect mice is very easily transmitted during mating. We propose to optimize this system to determine how well microbicides block sexual transmission of HIV-1. We shall also test in mice how the forms of HIV-1 that are widely distributed today can be controlled by microbicides. We have already shown that vaccination can reduce susceptibility to HIV-1 in mice. We also plan to both vaccinate mice and then treat with microbicides to determine if it is possible to completely prevent sexual transmission of the virus. Our hope is that the model of sexual transmission of HIV-1 in mice can accelerate the development of safe and effective microbicides that can be used to control the AIDS pandemic.